1. Field of the Invention
The present invention relates to a low pass filter, and more specifically, to a low pass filter with a large time constant.
2. Description of the Prior Art
The low pass filter is a common device in various types of circuits including a conventional servo loop 10 shown in FIG. 1. Generally speaking, the conventional servo loop 10 is used for providing a high pass filtering characteristic between an input signal Vi and an output signal Vo. To act as a high pass filter, the servo loop 10 is composed of a gain device 12 and a low pass filter 14 connected in a negative feedback configuration. As shown in FIG. 1, k/s represents a transfer function of the low pass filter 14, which includes a pole when s=0, while k is a constant corresponding to a time constant of the low pass filter 14.
Please refer to FIG. 2 showing a circuit diagram of the low pass filter 14. Node A is connected to the output voltage Vo in FIG. 1, and node B is fed back to the input voltage Vi in FIG. 1. The low pass filter 14 is composed of an operational amplifier 16, a capacitor 18 (with capacitance C1), and a resistor 20 (with resistance R1) connected in an integrator configuration. In this configuration, the transfer function of the low pass filter 14 is as follows:
                              k          s                =                  -                      1                                          sC                1                            ⁢                              R                1                                                                        formula        ⁢                                  ⁢        1            wherein C1R1 is the time constant of the low pass filter 14. Thus it can be seen by formula 1 that the constant k is inversely proportional to the time constant C1R1. That is, the larger the time constant is, the smaller k is. Therefore, for better high pass filtering characteristic of the servo loop 10, a larger time constant of the low pass filter 14 is preferable.
For a larger time constant, both the capacitor 18 and the resistor 20 of the low pass filter 14 need large values. However in an integrated circuit, a capacitor and a resistor having large values occupy too much area, so that the cost of the design increases accordingly.